When doing medical research with human subjects, which four limitaions are unavoidable

There are ethical or privacy concerns to consider.

Explanation:

Human subject research is a systematic, scientific investigation that can be either interventional, a trial research or observational (test).

It is necessary to follow the Nuremberg code (1948) and have a voluntary informed consent, the research should be based on previous good results, on prior research, physical or psychological injury and harm should be avoided, benefits should outweigh risks, proper experience of a researcher is needed, right to withdraw consent, and research must stop if harm is arising.

In 1964, a guideline for medical doctors in biomedical research has been established by WMA (The World Medical Association). Helsinki declaration stated the ethical principles involving human subjects that was adopted.

In, 1982  International Ethical Guidelines for Biomedical Research Involving Human Subjects by the World Health Organization (WHO) and Council for International Organizations of Medical Sciences (CIOMS) has been established.  

According to Guidelines (2002) ethical justification and scientific validity of research, ethical review, informed consent, vulnerability ¬ of individuals, groups, communities and populations, women as research subjects, equity regarding burdens and benefits, choice of control in clinical trials, confidentiality, compensation for injury, strengthening of national or local capacity for ethical review, and obligations of sponsors to provide health-care services are outlined.

There are 4 most important principles of ethics such as autonomy, beneficence, non-malfeasance and justice.

Ethical Review Board (ERB) is a committee that has been formally designated to approve, monitor, and review biomedical and behavioural research involving humans. It aims to protect the rights and welfare of the research subjects.  The ERB must have at least five members. The members must have enough experience, expertise, and diversity. If the ERB works with studies that include vulnerable populations, the ERB should have members who are familiar with these groups. The ERB must include at least one person who is not affiliated with the institution or in the immediate family of a person affiliated with the institution.  ERB members may not vote on their projects. The ERB may include consultants in their discussions to meet requirements for expertise or diversity, but only actual ERB members may vote.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).